The semiconductor industry has been using photolithography to build integrated circuits on silicon wafers for decades. In the patterning process of photolithography, a polymer film called a photoresist is deposited over a thin film of one of a variety of materials on a silicon wafer. Next, in an exposure tool, light of a very specific wavelength is projected through a pattern-bearing mask onto the photoresist. Regions of the photoresist exposed to the light undergo chemical changes, making them either more or less susceptible (depending on the process) to being removed in a subsequent chemical developing process. The pattern of the mask with various features is thus transferred to the photoresist. These features represent electronic components (e.g., contacts, channels, gates, wires, etc.). Through a subsequent process of chemical and/or physical etching, the pattern of the photoresist is then transferred to the underlying thin film. Multiple iterations of this thin-film patterning process, along with several other physical processes, produce integrated circuits.
Continuous reduction in sizes of the printed features has enabled miniaturization of electronic devices, exponential increases in functionality, and dramatic decreases in cost that characterize Moore's law. This has been facilitated by lowering wavelength of the exposure light from near ultraviolet (UV) to deep ultraviolet. Making the leap to deep ultraviolet would require dramatic materials innovations and a change in photoresist technology. The standard type of near-UV photoresist, known as the DNQ-Novolac resist, utilizes a hydrogen bonding-based interaction to control dissolution inhibition/acceleration of the novolac resin via the photoreaction of the DNQ molecule. This dissolution inhibition mechanism had been successfully applied as the primary imaging mechanism for nearly 30 years in G-line (436 nm) and I-line (365 nm) lithography before the problems in resist sensitivity were encountered in switching to 248 nm.
To address the photosensitivity issue, an entirely new breed of photoresist—chemically amplified photoresists—was developed. The chemistry comprises two main steps. First, a chemically amplified resist produces a catalyst through an initial photochemistry reaction upon exposure to light. The catalyst then interacts with the surrounding polymer matrix to pursue a cascade of chain reactions which can chemically enhance the resist imaging process. These chemically amplified photoresists have become the workhorse resist materials of the semiconductor industry for about 20 years, being utilized in both of the 248 nm and 193 nm photolithography technologies.
One example of a chemically amplified photoresist comprises a poly(4-hydroxystyrene) (pHOST) protected by an acid-labile tert-butoxycarbonyl (t-BOC) functional group and a triphenylsulfonium hexafluoroantimonate (TPS.SbF6), as shown in FIG. 3. The TPS.SbF6 is a photoactive compound which can generate acid after being exposed to light. The acid generation process is shown in FIG. 4A, in which the TPS.SbF6 molecule absorbs a photon and produces a strong hexafluoroantimonic acid molecule (H+SbF6−). This type of photoactive material is referred to as the photo acid generator (PAG). The acid generated by the photo acid generator then catalyzes the cleavage of the t-BOC groups in a post-exposure bake process as shown in FIG. 4B. The deprotection reaction not only changes the polymer from hydrophobic to hydrophilic, but also generates additional acid, catalyzing further t-BOC cleavages in a cascade of deprotection. The cascade of deprotection amplifies the effect of the photochemical event, yielding a great sensitivity. As opposed to hydrogen bond based interaction which regulates dissolution inhibition/acceleration in novolac/DNQ resists, the protection/deprotection scheme used in chemical amplification system provides resin polarity switch that leads to extraordinary solubility differences (more than 5-order dissolution rate difference in aqueous base solutions).
An exposed chemically amplified photoresist film may be developed with a positive-tone developer or a negative-tone developer. The former had been employed dominantly until recently. In a positive tone development application, the deprotected polymer is formed in the exposed regions of the film after a light exposure through a bright-field mask and a subsequent post exposure bake. An aqueous base solution (e.g., tetramethylammonium hydroxide (TMAH) solution) is used to wash away the deprotected polymer, leaving features on wafer resembling the pattern of the mask. While used successfully in several technology nodes, this process is facing challenges in printing small contacts and narrow trenches. In comparison, the negative tone development, using a non-polar organic solvent such as anisole to dissolve the protected polymer, enables better narrow trench patterning. This technique, along with other techniques such as resolution enhancement and double-patterning techniques, extends 193 nm-immersion lithography to at least the 14 nm node regime.
Various resolution enhancement techniques have been used to correct diffractive effects of light and to improve the fidelity with which the desired pattern is printed on the wafer. In most of the resolution enhancement techniques such as optical proximity correction (OPC), a simulation is made of how a feature will print on a wafer. The simulation is then used to adjust a pattern contained on a mask or reticle in a way that compensates for the expected distortions. As part of the simulation, a model is often used to predict how the light sensitive resist materials will behave when exposed with a particular mask pattern. Typically, simulations are conducted at sparsely chosen sampling locations or sites to reduce the overall processing time spent on simulation. The local values of image intensity and related other properties, such as derivatives, are calculated at these simulation sites to estimate the behavior of the resist at that point. Suitable site selection procedures are disclosed by, for example, U.S. Pat. No. 7,073,162, which is incorporated herein by reference, to ensure the best representation when a sampling approach is taken.
An alternative simulation technique, disclosed in U.S. Pat. No. 7,378,202 which is incorporated herein by reference, is grid-based. In this technique, image intensity values are calculated at a grid of points on the wafer, and the image intensity values are supplied to a resist simulator that produces a resist surface function. The resist surface function may be a linear combination of modeling terms. Each of the modeling terms may be a result of a sequence of operators applied to the aerial image. By thresholding of the resist surface function, a resist contour that estimates the pattern of features to be formed on a wafer may be obtained. This resist model is often referred to as Compact Model 1 (CM1) resist model.
While conventional resist simulators including those discussed above have been successfully employed to predict how features will print on a wafer through a positive-tone development process, these simulators could not provide a complete description of some photoresists in a negative-tone development process, resulting in poor matches between simulated and experimentally measured data. In particular, systematic errors occur with one-dimensional grating-type features and two-dimensional pillar-type features. One cause of the systematic errors is due to unique characteristics of the development rate for the negative tone development, e.g., the relatively fast development rate in regions exposed by light (minimum development rate) and the relatively slow development rate in regions unexposed by light (maximum development rate). It is desirable to develop a resist model that can account for effects of the large minimum development rate.